Electrical contact material containing silver,cadmium oxide,tin and cobalt

ABSTRACT

ELECTRICAL CONTACT MATERIAL IS DISCLOSED CONTAINING SILVER, CADMIUM OXIDE, TIN AND COBALT HAVING A MICROSTRUCTURE OF CADMIUM OXIDE PARTICLES DISPERSED IN A SILVER MATRIX WHICH ARE VERY FINE AND FIBROUS. THE FIBROUS STRUCTURE GENERALLY OCCURS NORMAL TO THE OXIDIZED SURFACE.

3,799,770 CADMIUM .L A m H n 09 A 01 D D A w R mu ....d 6 m u K H A O T March 26, 1974 4, ELECTRICAL CONTACT MATERIAL CONTAINING SILVER;

United States Patent 3,799,770 ELECTRICAL CONTACT MATERIAL CONTAINING SILVER, CADMIUM OXIDE, TIN AND COIfALT Takeo Harada, Mishima-shi, and. Takeshi Monya and Rikichi Wakasa, Susonomachi, Shizuoka-ken, Japan, assignors to P. R. Mallory & Co., Inc., Indianapolis,

Ind.

Filed Dec. 6, 1971, Ser. No. 205,249 Claims priority, application Japan, Dec. 7, 1970,

45/107,588 Int. Cl. C22c 5/00 US. Cl. 75173 R Claims ABSTRACT OF THE DISCLOSURE Electrical contact material is disclosed containing silver, cadmium oxide, tin and cobalt having a microstructure of cadmium oxide particles dispersed in a silver matrix which are very fine and fibrous. The fibrous structure generally occurs normal to the oxidized surface.

BACKGROUND OF THE INVENTION The present invention relates tonew and useful improvements in a silver-cadmium oxide electrical contact material used for electrical contacts.

In general, silver-cadmium oxide electrical contact materials are characterized by being more resistant to sticking and resistant to erosion than silver electrical contact material, and the contact resistance is stable, so they are used widely for light current loads and medium-heavy loads.

Such silver-cadmium oxide electrical contact materials are manufactured by a powder metallurgical process, by a post oxidation process, or by a pre-oxidation process. The powder metallurgical process is a process 1n which a moulded mixture of silver powder and cadmium oxide powder is sintered, as described in greater detail, for example in US. Pat. 2,145,690. The post oxidationprocess comprises preparing a silver-cadmium alloy, working it to desired shape and heating it in oxidizing atmosphere so as to have cadmium thereof selectively internal oxidized. Further information may be found in US. Pats. 2,539,298 and 2,673,167. The pre-oxidation process, described in US. Reissue Pat. 27,075 and U.S. Pat. 3,545,067 involves forming fine particles of silver-cadmium alloy, for example by atomizing a melt of silver-cadmium alloy, internally oxidizing the cadmium in the particles, consolidating the oxidized particles and mechanically working to desired shape.

However, silver-cadmium oxide electrical contact materials are often unsatisfactory in regard to resistance to erosion and in breaking characteristics.

Therefore an object of the present invention is to provide an improved electrical contact material having a lower erosion rate.

Another object of the present invention is to provide an electrical contact material having improved hardness.

Another object of the invention is to provide an electrical contact material having improved hardness at elevated temperature. v 7

Another object of the present invention is to provide a contact material having lower sticking power.

Another object of the invention is to provide a contact material which has a stable contact resistance.

Other objects will be apparent from the following description and drawings wherein:

FIG. 1 is a photomicrograph of thejmicrostructure of the electrical contact material of the present invention at 200 magnifications, containing 10% CdO, 0.36% Sn and 0.01% Co.

3,799,770 Patented Mar. 26, 1974 FIG. 2 is a photomicrograph of the microstructure of post-oxidized prior art contact material containing silver and cadmium oxide at the same magnification as FIG. 1 and having the same cadmium oxide composition as the material of FIG. 1.

FIG. 3 is a photomicrograph of electrical contact material of the prior art containing, silver, cadmium oxide and tin at the same magnification as FIG. 1 with the same composition of cadmium oxide and tin as the material of FIG. 1.

SUMMARY OF THE INVENTION The electrical contact characteristics of silver-cadmium oxide electrical contact material are improved by adding about 0.2 to about 5% tin and about 0.005 to 0.5% cobalt to the material. The microstructure comprises fine and fibrous grains of cadmium oxide precipitated in a silver matrix. Preferably the material contains about 0.3 to 2% tin and about 0.01 to 0.3% cobalt.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrical contact material manufactured by oxidizing a silver-cadmium alloy prepared by adding 0.2-5% tin (weight percent and hereinafter in this application) and 0.005 to 0.5 cobalt to silver-cadmium to provide a material consisting essentially of up to 30% cadmium, about 0.2 to about 5% tin, about 0.005 to about 0.5 cobalt, the balance silver.

The addition of tin has the effect of causing the configuration of the grains of cadmium oxide during selective oxidation to be a fibrous structure normal to the oxidized surface as shown in FIG. 3. The added amount of tin under 0.2%, however, does not make the effect of the addition distinguishable and an amount over 5% increases the precipitation of cadmium at the grain boundary, reducing the electrical conductivity and workability of the alloy considerably. Preferably the tin content is about 0.3 to about 2%.

The addition of cobalt is effective in causing the grains of cadmium oxide after the internal oxidation of the silver-cadmium alloy to be remarkably fine as can be seen from comparing FIGS. 1 and 3. The cadmium oxide particles are generally not larger than 1,u x 10a. The added amount of cobalt under 0.005% does not make the effect of the addition distinguishable. Cobalt, however, is only slightly solid soluble in silver and cadmium, so its added amount is not above about 0.5 Preferably the cobalt content is from about 0.01 to about 0.3%. The preferred material consists essentially of about 5 to about 15% cadmium, about 0.3 to about 2% tin, about 0.01 to about 0.3% cobalt, the balance silver.

The electrical contact material of the present invention may be manufactured for example by either the postoxidation or pre-oxidation techniques mentioned above.

In regardto the pre-oxidation technique, U.S. Reissue Pat. 27,075 and US. Pat. 3,545,067 are hereby incorporated into the present application by reference and may be referred to for exemplary processing parameters which may be used in processing according to this technique.

In utilizing this technique the cobalt and tin may be added to the melt prior to atomizing. An excess of tin and/or cobalt may be added to allow for losses of tin and/or cobalt while adding these elements to the melt and/or during pouring.

The tendency for the fibrous structure to appear normal to the oxidized surface is often lost in this technique. While the fibrous structure is normal to the oxidized surface within individual particles, since these particles are randomly consolidated and are subsequently mechanicalout the microstructure.

brous structure to be normal to the oxidation face is not lost through consolidation of oxidized particles and/or mechanical working to shape.

In general, in oxidizing the fine particles according to the pro-oxidation technique, or the finished or semifinished shape according to the post-oxidized technique, an oxidation temperature of about 700 to about 850 C. and a time of about to about 150 hours is preferred, depending upon section size, oxidizing temperature, and oxygen content of the atmosphere.

The hardness of the electrical contact material is greater than prior art material of similar cadmium oxide composition without cobalt. It is really at least about higher at room temperature and this difference increases at elevated temperature. For example at 400 to 700 C. the hardness is generally at least about twice as hard as prior art material of similar cadmium oxide composition.

Also the arc erosion rate and sticking power are lower with the material of the present invention. The are erosion rate is usually at least 25% lower and often 50% or more lower. The sticking is usually at least 50% lower and is often two times lower.

SPECIFIC EMBODIMENTS An electrical contact material is manufactured by adding 8.59% cadmium, 0.5% tin, 0.5% cobalt to a silver melt to form a casting containing 8.6% Cd, 0.36% Sn and 0.02% Co, rolling the casting to sheet having the thickness of 1.5 mm., and then heating it at 700 C. for 96 hours in an oxygen containing atmosphere.

The electrical conductivity and hardness at room and elevated temperature of said silver-cadmium oxide alloy are shown in Table 1, together with those of another alloy as a comparative material prepared under the same conditions as the silver-cadmium oxide-tin-cobalt alloy by adding 9.0% cadmium to silver.

N orE.A=The contact material according to the present invention. B =The comparative material.

The hardness of the contact material according to the present invention is improved and is about two and a half times as large as that of the comparative material at elevated temperatures because the addition of tin and cobalt causes the grains of cadmium oxide to be fine and fibrous.

The results of the erosion test carried out with the contact material according to the present invention and the comparative material in the shape of a contact having the size of 7 mm. x 7 mm. x 1.2 mm. assembled in a 35 ampere frame Magnetic Contactor are shown in Table 2.

NorE.-A=The contact material to the present invention. B=The comparative material.

Test conditions:

Voltage: 200 v. Current:

at making, 47 A., pf. 0.35 at breaking, 235 A. pf. 0.65 Switching frequency: 20 time/ min.

Moreover, the results of the stick test carried out with the contact material according to the present invention and the comparative material respectively in the shape of a contact having diameter of 6 mm. and a thickness of 1.5 mm. are shown in Table 3.

TAB LE 3 Number of Sticking Eroded times of power amount Sample switching (kg) (mg.)

A 10 l. 5 l0. 0 B I 15.0 35. 0

1 Stuck at 5th time. Determined when contact stuck at 5th time of The amount of the contact material eroded according to the present invention is about half that of the comparative material (not containing tin and cobalt) as shown in Table 2, and Table 3 shows that the sticking power of the material of the present invention is about one-third that of the comparative material in the stick test under the current 2000 a.

Thus, the contact material according to the present inyention has improved hardness erosion characteristics and antisticking properties which make it industrially very favorable.

What is claimed is:

1. An Ag-CdO type electrical contact material having improved hardness and erosion rate, the material consisting essentially of an effective amount up to 30 wt. percent Cd, about 0.2 to about 5 wt. percent Sn, about 0.005 to about 0.5 wt. percent Co, and the balance Ag, the material being oxidized to convert oxidizable constituents to their respective oxides prior to use as an electrical contact material.

2. The Ag-CdO type electrical contact material of claim 1, wherein the Cd content is from about 5 to about 15 wt. percent, the Sn content is about 0.3 to about 2 wt. percent, and the Co content is about 0.01 to about 0.3 wt. percent.

3. The Ag-CdO type electrical contact material of claim 1, wherein the microstructure of the contact material comprises CdO grains in an Ag matrix in which the CdO grains are fine and fibrous.

4. The Ag-Cdo type electrical contact material of claim 3, wherein the fibrous structure of the CdO grains is substantially normal to the oxidized surface of the contact material.

5. The Ag-CdO type electrical contact material of claim 4, wherein the CdO particle size is not generally larger than I p x 10;.

6. The Ag-CdO type electrical contact material of claim 2, wherein the Sn content is greater than about 0.5 wt. percent but less than about 2 wt. percent.

7. A method of making the Ag-CdO type electrical contact material of claim 1 comprising the steps of providing a melt of Ag-Cd-Sn-Co to form a body consisting essentially of 5 to 30 wt. percent Cd, 0.2 to 5 wt. percent Sn, 0.005 to 0.5 wt. percent Co, the balance Ag, and subjecting the body to oxidation to convert oxidizable constituents to their respective oxides and form the contact material, the Sn and Co constituents cooperating to cause grains of CdO to be fine and fibrous.

8. The method of claim 7, including the further step of causing grains of Cd() to be substantially normal to the oxidized surface of the contact material.

9. The method of claim 7, wherein the body is subjected to oxidation prior to forming a contact shape.

10. The method of claim 8, including the further step of forming the body into a cast shape having the thickness of the contact material prior to subjecting the cast shape to oxidation.

11. The method of claim 8, wherein the CdO particle size is about 1 x 101.4 or less.

12. The method of claim 10, wherein Cd-Sn-Co are added to a melt of Ag.

13. A method of making the Ag-CdO type electrical contact material of claim 2 comprising the steps of forming an Ag melt, adding to the Ag melt Cd-Sn-Co to form a body consisting essentially of 5 to 15 wt. percent Cd, 0.3 to 2 wt. percent Sn, 0.01 to 0.3 wt. percent Co, the

6 balance Ag, subjecting the body to oxidation to convert the oxidizable constituents to their respective oxides, and causing the grains of CdO to be fine and fibrous and norme] to the oxidized surface thereby forming the electric contact material.

14. The method of claim 13, wherein the CdO particle size is about 1,7 1: x 10por less.

15. The method of claim 14, wherein the Sn is more than about 0.5 wt. percent but less than about 2 wt. percent.

References Cited UNITED STATES PATENTS 2,932,595 4/1960 Pflumm -173 -R 3,607,244 9/ 1971 Kabayama et a1. 75-173 R 2,796,346 6/1957 Stumbock 75-173 R L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner 

